The determination of gas and liquid flow rates in gas-liquid fluid mixtures is important in the oil and gas industry.
An example of an apparatus for measuring such flow rates is Schlumberger's Vx™ system (see e.g. I. Atkinson, M. Berard, B.-V. Hanssen, G. Ségéral, 17th International North Sea Flow Measurement Workshop, Oslo, Norway 25-28 Oct. 1999 “New Generation Multiphase Flowmeters from Schlumberger and Framo Engineering AS”) which comprises a vertically mounted Venturi flow meter, a dual energy gamma-ray hold up measuring device and associated processors. This system allows the simultaneous calculation of gas, water and oil volumetric flow rates in multi phase flows.
However, with conventional implementations of Vx™ technology the accuracy of the calculations starts to degrade as the gas volume fraction (GVF) increases above about 90%. In particular, at high GVF it can be difficult to determine properties of the liquid phase.
WO2005/031311 discloses a flow conditioner for providing an isokinetic main fluid stream to be sampled by a sampling probe. The flow conditioner is a pipe section which includes a swirl inducing section, a flow straightener and an orifice plate. The swirl inducing section distributes a liquid film on the wall of the pipe section which is shed back into the gas core by the orifice plate, whereby turbulence causes good mixing of the liquid and gas phases. Straightening the flow of the gas core prior to the orifice plate reduces the amount of liquid that is redeposited onto the pipe wall after shedding by the orifice plate upstream of the sampling point.
WO 2004/106861 proposes a multi phase flowmeter in which a twisted tape device is used to separate a liquid phase from a liquid-gas mixture into an annular film on the tube wall of a straight pipe section. Measurements are made to determine the liquid flow rate in the film. Following the straight pipe section, the liquid is re-entrained into the gas stream by an expansion contraction system. The homogenised flow is then passed to a Venturi.
J. Gibson and M. Reader-Harris, Swirling flow through Venturi tubes of convergent angle 10.5° and 21°, Proc. FEDSM2006, Jul. 17-20 2006, Miami, Fla., discusses computations of swirling flows of dry gases in Venturi tubes.